The present invention relates to improvements in the field of are welding. More particularly, the invention is concerned with a method and apparatus for controlling weldpool penetration during poor pass welding in open butt joints.
On of the most common type of welds found in industry, especially for industrial pipe fabrication, is the open butt joint weld. Such a type of weld is most often a multipass weld since more than one pass of the welding torch over the workpiece joint is generally required to fill the joint, but the first or "root" pass is by far the most difficult to perform and only experienced certified welders are able to achieve good quality root pass welds having adequate penetration. Precise centering of the welding torch in the joint and critical control of the welding parameters, i.e. heat and mass balance, are required to maintain an adequate penetration of the weldpool. Tolerances for weld penetration are particularly stringent: too much penetration will obstruct the flow in the pipe while too little leaves stress raising defects that dramatically reduce the fatigue life of the joint. Thus, one can see that automation of this type of welding presents a major problem, namely weldpool penetration control.
Means for controlling weld penetration have already been proposed, for instance in U.S. Pat. Nos. 3,299,250 and 3,567,899. According to these patents, the depth of penetration of the weld is measured indirectly by means of infrared sensors which sense the infrared radiation emanating from the molten metal at the weld. The heat flow characteristics of the weld as monitored by the infrared sensors are then processed to provide control signals for controlling the current applied to the welding torch. Such weld penetration control systems, however, are applicable only to closed butt joints such as U-shaped or square butt joints, where mechanical forces play a very little role, if any, and where geometry is constant and the heat flow is the determining factor. In an open butt joint, the weldpool is in a mechanical equilibrium that changes much more rapidly than temperature and is affected by many other factors, e.g. gap, alignment, root face, etc.